1. Field of the Invention
The present invention relates to a method of slicing a semiconductor single crystal ingot with a wire saw slicing apparatus and a semiconductor single crystal wafer sliced by the method.
2. Description of the Related Art
There is known a wire saw slicing apparatus as a means for slicing brittle materials such as compound semiconductor crystal ingots and silicon semiconductor crystal ingots. The wire saw slicing apparatus, as shown in FIG. 4, includes three plastic main rollers 10A, 10B and 10C of the identical construction disposed with their axes parallel spaced from one another, and a wire 12 wound spirally around helical grooves 14a, 14b and 14c formed at regular intervals or pitches in the respective outer peripheral surfaces of the main rollers 10A-10C. The main rollers may be plural in number and should by no means be limited to any particular number, but four or three main rollers as in the illustrated embodiment are used in general. The main roller 10C constitutes a drive roller and is connected in driven relation to a drive motor 16. A rotary motion of the main roller 10C is transmitted via the wire 12 to the remaining main rollers 10A, 10B which constitute driven rollers.
The wire 12 has one or a leading end portion wound around a wire reel bobbin 22 via a tension adjustment mechanism 20. The wire reel bobbin 22 is rotatably driven by a torque motor 24. A tension on a portion of the wire 12 extending between the tension adjustment mechanism 20 and the wire reel bobbin 22 is regulated according to a voltage applied to the torque motor 24. And, a tension on a portion of the wire 12 running between the tension adjustment mechanism 20 and the drive roller 10C is adjusted at a constant value by the tension adjustment mechanism 20.
Similarly, the opposite or a trailing end portion of the wire 12 is wound around a wire reel bobbin 32 via a tension adjustment mechanism 30. The wire reel bobbin 32 is rotatably driven by a torque motor 34. A tension on a portion of the wire 12 extending between the tension adjustment mechanism 30 and the wire reel bobbin 32 is regulated according to a voltage applied to the torque motor 34. And, a tension on a portion of the wire 12 running between the tension adjustment mechanism 30 and the drive roller 10C is adjusted at a constant value by the tension adjustment mechanism 30.
A workpiece 40 is composed, for example, of a semiconductor single crystal ingot having an orientation flat and attached by bonding to a workpiece holder 42 via the orientation flat. The workpiece holder 42 is vertically moved up and down along a linear path.
The wire saw slicing apparatus of the above construction operates as follows. The drive roller 10C is rotated by the drive motor 16 to reciprocate the wire 12 in the axial or longitudinal direction thereof. A working fluid containing abrasive grains is supplied to a contact area between workpiece 40 and the wire 12. While keeping this condition, the workpiece 40 is further moved downwards whereby the workpiece 40 is sliced at one time into a multiplicity of wafers by a lapping action attained by the reciprocating wire 12 and the abrasive-grains containing working fluid supplied thereto.
It is known that a semiconductor single crystal cracks or cleaves in a fixed direction to form a smooth face, that is, a cleaved face. This cracking direction is called a cleavage direction which varies with the kind of the crystal.
For example, as shown in FIGS. 7 to 9, in case of a silicon single crystal (W), a plurality of cleavage directions (A) exist according to crystal orientations. FIG. 7 shows cleavage directions of a (100) silicon single crystal, FIG. 8 shows those of a (110) silicon single crystal and FIG. 9 shows those of a (111) silicon single crystal.
Conventionally, when a semiconductor single crystal ingot such as a silicon semiconductor single crystal ingot (hereinafter, may be merely referred to as "ingot") is sliced by the wire saw slicing apparatus, the slicing operation was conducted with the cleavage direction of the silicon single crystal ingot almost corresponding with the wire running direction.
For example, in case of slicing a (100) silicon single crystal ingot, as shown in FIGS. 5 and 6, first a back plate 41 is adhered to the orientation flat portion (OF) of the ingot (W), and then the adhered back plate 41 is adhered to the workpiece holder 42 (FIG. 5), or first the back plate 41 is adhered to the portion rotated or shifted by 90.degree. from the orientation flat portion (OF) of the ingot (W), and then the adhered back plate 41 is adhered to the workpiece holder 42 (FIG. 6). Thereafter, the ingot (W) adhered to the holder 42 is moved down and pressed against the wire 12 of the wire saw slicing apparatus.
In this case, there are two cleavage directions (A.sub.1, A.sub.2) which are normal to each other when seen in the cross-section along the radial direction. In the (100) silicon single crystal, the orientation flat portion (OF) is mostly formed in either one of the two cleavage directions (A.sub.1, A.sub.2). With either one of the two cleavage directions (A.sub.1, A.sub.2) corresponding with the running direction (Y) of the wire 12, the ingot (W) is sliced.
The procedure of slicing the ingot (W) by the conventional wire saw slicing apparatus is described with reference to FIG. 4.
First, an ingot (W) is prepared (step 1). Next, the crystal orientation in the distal end face of the prepared ingot (W) is measured (step 2). A back plate 41 is adhered to the orientation flat portion (OF) or the portion rotated or shifted by 90.degree. from the orientation flat portion (OF) of the ingot (W) (step 3). The back plate 41 adhered to the ingot (W) is further adhered to the workpiece holder 42 (step 4). Then, the ingot (W) which is incorporated with the back plate 41 and the workpiece holder 42 is secured to an attaching base 44 of the wire saw slicing apparatus (step 5). The attaching angle of the ingot (W) is adjusted in accordance with individual standards (step 6). Next, with the wire saw slicing apparatus, the ingot (W) is sliced to the central portion of the back plate 41 to produce a large number of sliced wafers (step 7). Thereafter, the ingot (W) is removed from the attaching base 43 of the wire saw slicing apparatus, with a large number of the sliced wafers being still adhered to the workpiece holder 42 (step 8). The removed ingot is soaked in hot water to separate a large number of the sliced wafers from the workpiece holder 42 (step 9). The separated wafers are cleaned to be as-cut wafers (step 10).
In the above-mentioned manner, as-cut wafers are prepared from the ingot (W). However, when the ingot (W) is sliced by the wire saw slicing apparatus, the traces of running of the wire are left as saw marks on the surface of each wafer with a result that damaged layers are formed along the saw marks. The damaged layers lead to occurrence of cracks along the cleavage directions in the sliced single crystal wafer by the wire vibration or the like effect. Thus, in the conventional slicing method, the sliced wafer is disadvantageously apt to be cracked because the saw marks run in accord with either one of the cleavage directions.